feat(client-compat): add explicit legacy HTTP+SSE compatibility

[kauandotnet]

Context

The built-in transport definitions intentionally cover Streamable HTTP and stdio. Some legacy endpoints still require HTTP+SSE, but compatibility must not mask authentication, network, or modern protocol failures.

Scope

• Add an explicit legacy SSE transport definition or isolated compatibility subpath.
• Offer opt-in Streamable HTTP to SSE fallback only for clear transport-mismatch responses.
• Construct a fresh official client and transport for every fallback attempt.
• Never fall back after generic network errors, authorization failure, OAuth failure, or an established modern exchange.
• Integrate typed failure classification, abort, reconnect policy, and deterministic shutdown.
• Emit a compatibility warning and document the deprecation posture.
• Keep legacy implementation code out of the default client entrypoint when practical.

Acceptance criteria

• Legacy SSE is never selected without explicit configuration or explicit fallback opt-in.
• Fallback preserves the primary failure when the fallback also fails.
• No credentials or authorization state cross an origin or issuer boundary.
• A retired or aborted primary attempt cannot publish the fallback client.
• Tests cover mismatch, authentication, outage, dual failure, abort, and cleanup.
• Modern Streamable HTTP behavior remains unchanged.

Dependencies

• Blocked by feat(client): expose typed runtime events and a stable failure taxonomy #12
• Blocked by feat(client): add upstream availability policies and explicit recovery APIs #14

Non-goals

• Making legacy SSE the default transport.
• Retrying every failed HTTP connection with GET.
• Session resumption or event replay.